Clinicians and researchers are often hampered in developing sound and practical remedies when working with any population that is understudied. This is especially true when there is little valid and reliable information upon which to build a reasoned approach to treat basic physical impairments. Efficient locomotion, which is foundational to the ease of life, has been well-documented in able-bodied children, but remains poorly understood in cerebral palsy (CP). Research has shown that a large percentage of adults with CP who stop walking do so because of physical fatigue and an inability to ambulate without tiring. Hence, a clinical need exists to develop and test rehabilitation strategies aimed at reducing the cardiopulmonary strain that accompanies walking before the child with CP enters adulthood. Against this backdrop, the primary goal of this project is to determine the impact of an aquatics-based, treadmill walking program on locomotor energetics and functional mobility in youth with spastic diplegia. Our first specific aim is to document changes in the aerobic function of spastic diplegic children following a 10-week program of underwater treadmill training (UTT). To accomplish this aim, 30 children with spastic diplegia will undergo UTT while another 30 spastic diplegic children serve as age- and sex-matched controls. We hypothesize that children undergoing UTT will demonstrate an increase in maximal aerobic power, a decrease in submaximal energy use, and an improvement in treadmill walking performance. We also predict that an increase in the ratio of lower-extremity muscle strength to total body mass will underlie anticipated improvements in locomotor economy resulting from UTT. Our second specific aim is to determine the clinical significance of enhanced aerobic function and relative leg strength as they relate to community mobility in the child with spastic diplegia. This aim will be addressed by quantifying training-related changes in daily step activity, preferred and maximal walking speeds, and distance covered in a timed walking test. Achieving our two specific aims will enable us to establish the aerobic and strengthening benefits of UTT and evaluate the synergistic effects of these adaptations on functional mobility. Our ultimate objective in this work is to develop and implement the most effective methods of reducing energy-wasteful movement in youth with CP. By accomplishing this goal, we hope to increase the likelihood that optimal function can be maintained as the child with spastic cerebral palsy transitions into adolescence and adulthood.